The present invention relates to centrifugal pumps in general, and more particularly to improvements in centrifugal pumps of the type wherein the casing includes several sections, especially a volute-type inner section and a cylindrical outer section or shell. Centrifugal pumps of the just outlined character can be used with advantage as feed pumps in power plants or the like.
It is already known to provide a high pressure centrifugal pump with a casing which includes a volute-type inner section and an outer section or shell which surrounds the inner section. Such casings are often used in multistage centrifugal pumps which are installed in power plants or like institutions wherein the pumps must deliver a hydraulic fluid at elevated pressure. The provision of a twin- or multiple-component casing contributes to the safety of operation. The shell surrounds all pressure generating and fluid conveying components including the aforementioned volute-type inner section. As a rule, the inner section of a conventional centrifugal pump with a multiple-component casing is welded to and thus permanently connected with the outer section or shell.
It is further known to use a composite or multi-component casing in connection with single-stage pumps. In such pumps (reference may be had to German Offenlegungsschrift No. 2,640,886), the volute-type inner section serves as a guide means for the fluid. Here, again, the inner section which guides and conveys the fluid is welded to the outer section or shell of the pump casing.
The just discussed conventional centrifugal pumps with multi-component casings wherein an outer component surrounds an inner component exhibit a number of serious drawbacks.
For example, if the volute-type inner section of the casing is damaged (e.g., owing to cavitation), the permanent (welded) connection between the inner and outer sections of the casing presents many problems preparatory to gaining adequate access to the inner section for the purposes of repair, i.e., the welded seam or seams between the inner and outer sections must be destroyed and reestablished when the repaired inner section is to be reinstalled in the outer section of the casing.
Another serious drawback of the aforediscussed conventional casings is that the making of welded seams between the inner and outer sections of the pump casing presents many problems. The locations of such seams are not readily accessible and the configuration of the seams is often extremely complex so that they cannot be formed by resorting to available automatic welding equipment. For example, a seam must be formed between the tubular elements through which fluid flows from the exterior of the outer section into the interior of the inner section or vice versa. This is normally accomplished by providing the outer section with an integral inwardly extending nipple which is welded to the inner section. The making of such nipple and its bonding to the inner section contribute significantly to the cost of the pump casing. Moreover, the quality of the welded seam or seams between the inner and outer sections of the casing must be tested with a high degree of accuracy. This also presents many problems because it is difficult to properly position the testing equipment (e.g., a source of X-rays and X-ray film) with reference to the seam or seams. The same holds true if the testing equipment employs other sources of penetrative radiation and one or more transducers which ascertain the percentage of radiation that is absorbed by the material of the seam and/or by the adjacent material of the inner or outer section of the casing.
The number of materials which can be used to make the outer section of the casing in a conventional pump is rather limited because the material must be such that the outer section can be readily and reliably welded to the inner section. As a rule, the quality of the metal (e.g., an alloy) which is used to make the inner section is higher than the quality of the metal which is used to make the pipes serving to deliver fluid to or remove fluid from the centrifugal pump. The quality of the material of the outer section of the casing is somewhere between the qualities of the material of the inner section and the material of the piping. The term "quality" is intended to denote in particular the weldability of the materials of various components of the pump and of the parts which are connected to the pump casing. The quality of the material of the inner section of the casing must be high in order to enable the inner section to stand long periods of use in spite of the very high speed of the fluid that flows therethrough.
The presence of welded seams between the inner and outer sections of a composite or multi-component pump casing is undesirable when the temperature of the conveyed fluid is very high or very low and the heat expansion coefficient of the material of the inner section is different from that of the material of the outer section. Unequal expansion and resulting distortion of the components of the casing can lead to pronounced shifting of the stator relative to the rotor of the centrifugal pump with adverse effects upon the reliability and safety of operation. Moreover, the wear upon the parts increases in response to pressure- or heat-induced distortion which cannot be compensated for by at least some movement of the inner section with reference to the outer section of the casing or vice versa.